1. Field of the Invention
The present invention relates to an improvement in a method and apparatus for scanning light beam.
2. Description of the Prior Art
A xerographic apparatus which records images such as characters, numerals or patterns by irradiating light beam on a charged photosensitive substance, such as a laser printer is well known. The present invention relates to an improvement in method of irradiating the light beams on the photosensitive substance for improving picture quality by controlling diameters of the beam spots irradiated on the photosensitive substance.
FIG. 1 shows a sectional view showing an example of general structural configuration of the xerographic apparatus, which comprises a photoreceptor drum 1 rotating around a shaft 101, a pre-charger 2 for donating charges by a corona discharging onto the photoconductive layer 102 at the surface of the photoreceptor 1, a semi-conductor laser 3 capable of emanating modulated light, a collimeter lens 4 for making parallel light from the output light of the laser 3, a rotary polygonal mirror 5 driven by a motor 51, a focusing lens 6 for focusing light beam on the photoconductive layer on the photoreceptor 1, a developing means i.e., a tonor donating means 7 for donating negative-charged toner on the photoconductor layer 102, a transferring charger 8 for transferring toner from the photoreceptor onto the surface of a recording sheet 9, a mechanical toner cleaner 10 and an optical decharging lamp 11 for removing residual charge and toner. The apparatus of FIG. 1 operates as follows:
The semiconductor laser 3 emanates light modulated by a known measure of stop and feed laser current. The light passes through the collimator lens 4, is reflected by the rotary polygonal mirror 5, and through the converging lens 6 sweepingly irradiates the photoconductor layer 102 on the surface of the photo-receptor 1. By the rotation of the polygonal mirror 5, the light beam spot on the photoconductive layer 102 scans very fast widthwise of receptor drum 1, that is the direction of the sweeping of the light beam spot is substantially parallel to the axis of the photo-receptor drum 1. This widthwise scanning is hereinafter referred as main scanning, and a secondary scanning or sub-scanning is made by relative movement of the photoreceptor drum 1 around its axis, that is in the right angle to the main scanning line of light beam spot. The main scanning and the sub-scanning are represented as shown by FIG. 2, which schematically shows a small part of the surface of the photoconductive layer 102. At the exposure to the irradiation of light, the precharged photoconductor layer 102 loses charge at the light-exposed parts, thus forming latent image of charge on the photoconductor layer 102. The latent image is then developed by known tonor device 7 which for example comprises known magnetic brush.
In actual recording by the above-mentioned xerographic apparatus, due to slight differences between vertical angles of mirros of the polygonal mirror, the vertical positions of the scanning lines are likely to fluctuate up and down, thereby making the vertical pitches of the scanning lines uneven. Due to this uneveness of the subscanning, in case of positive developing, wherein parts not exposed to the light beam spots are developed black, such uneveness of the sub-scanning makes undesirable black horizontal lines as shown in FIG. 5 which is not in the original image.